An K12 strain, J5--3 (R313), was presumed to carry an R factor, R313, which determined resistance to the drugs tetracycline (Tc), streptomycin (Sm) and Sulphonamide (Su), and also determined a host specificity, hsII. When this strain was used as a donor of drug resistance, separation of R factor-carried determinants was observed in ex-conjugants. Resistance to Tc and hsII character were not separated, nor was resistance to Sm separated from resistance to Su, but separation of these two pairs of characters was observed. Tetracycline-sensitive segregants, obtained by penicillin selection, were resistant to Sm and Su, but had lost the hsII character. Similarly, segregants for selected sensitivity to Su were sensitive to Sm, but resistant to Tc and hsII+. The same pattern of separation of characters was also observed when drug resistance was transduced with phage PI, with the additional finding that the Sm and Su resistant trans-ductants lacked sex factor activity. The resistance to Sm carried by these transductants could be mobilized by an R factor, R 143. Explanations of this behaviour are considered, including the possibility that the strain J5--3 (R313) had carried two R factors. This explanation would also require that the transduction of an R factor by phage PI is not always complete.


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